Inside reactor buildings, neutrons can have energies ranging over 9 orders of magnitude, from 25 meV (thermal energies) to 20 MeV. Neutrons interact with nuclei of the atoms inside the human body and may present a significant health risk to workers working in environments where these neutral particles are found. Neutrons can also be found at accelerator sites and around natural and man-made radiation sources.
For greater worker safety, radiation safety officers must, on a regular basis, characterize the neutron fields inside the nuclear power plants using elaborate and heavy instruments. The measurements provided by these instruments are essential in mapping “hot” areas inside a reactor building and can assist in determining if the personal dosimeters in service fulfill adequately the monitoring needs. The amount of biological damage suffered by an exposed individual is dependent on the energy per unit tissue mass imparted by the incoming neutrons, i.e. the radiation dose, multiplied by a weighting factor that depends on the incident neutron energy. The product of the dose and the weighting factor is called the dose equivalent. Because the weighting factor can range from 1 to 20 it is imperative that the neutron energy be measured if the dose equivalent cannot be measured directly.
Many neutron detection and dosimetry techniques and neutron energy characterization methods have been devised over the years. The following paragraphs provide a description of prior attempts to address the problem of determining the neutron spectra. These include simple thermal neutron counters, neutron dosimeters and neutron spectrometry systems.
Simple thermal neutron counters are devices that count the low energy neutrons with a certain efficiency. These devices are commercially available and any of them can be used as part of the invention. Included in the category of “simple” neutron counters are the following three examples. These describe the best available options to be used in the invention because of their high neutron counting efficiencies. However others are possible. The prior art on simple neutron counters includes the following.
U.S. Pat. No. 3,102,198 to Bonner (1963) describes the now commonly used and commercially available Helium-3 gas proportional counter. This proportional counter uses Helium as a gas but enriched with the Helium-3 isotope. This gas offers a high detection efficiency for thermal neutron through the 3He(n,p)3H nuclear reaction.
Boron trifluoride is also used as a gas in a proportional counter similar to the one described above. The gas is designated as 10BF3, as it is highly enriched in 10B. This isotope of Boron offers a high detection efficiency for thermal neutrons through the 10B(n,α)7Li reaction.
6LiI(Eu) is a scintillator that detects thermal neutrons through the 6Li(n,α)3H reaction with a high efficiency. Energy from the nuclear reaction is converted to light photons and detected with a light detector.
Prior art on neutron dosimeters methods include the following. The Anderson-Braun and Leake detectors are two types of neutron dosimeter for the measurement of the dose equivalent without resorting to the prior measurement of the neutron energy spectrum. They consist of a thermal neutron detector surrounded by a shell of moderator, usually made of plastic such as polyethylene, of about 25 cm in diameter. Incident energetic neutrons are slowed down through collisions with the hydrogen atoms present in the moderator. When they reach thermal energies, the neutrons are then efficiently detected by the thermal neutron detector at the centre of the moderator. The device is a useful dosimeter in the range of 25 meV to 20 MeV but presents an inaccuracy in response of up to a factor of 5. Such instruments are calibrated to give neutron dose equivalent but do not provide neutron energy information. The size of the moderator is fixed.
The tissue equivalent proportional counter, the so-called Rossi-counter, is a spherical proportional counter, usually of 5 to 12 cm in diameter, whose external shell is made of conductive plastic and which is filled with a counting gas that has nearly the same atomic composition as human muscle. Incident neutrons interact with the walls of the detector and secondary charged particles enter the gas and their specific energy loss is measured. This device is an “energy loss spectrometer” which provides no information on the incident neutron energy but which allows the neutron dose equivalent to be determined for neutron of energies in excess of 100 keV.
U.S. Pat. No. 5,278,417 to Sun describes a spherical detector surrounded by perforated shells of different types of moderator (polyethylene, lead and borated polyethylene) to allow the spherical dosimeter to provide dose equivalent in the GeV range of energies. All shells are all present at once and are not removable. The device is also not a neutron energy spectrometer.
Prior art on spectrometry systems include the following. Hing et al. describe a proton recoil spectrometer, a transportable instrument consisting of one or more gas detectors, which deduces the neutron energies from the energy imparted to protons of the counting gas inside the detector. This system provides very good energy resolution of the neutrons. However, it responds only to neutrons above a few 10's of keV and the sensitivity is lower than that of a thermal neutron counter surrounded by a moderator. From the energy spectrum, other quantities of interest, such as the dose equivalent, can be found using conversion factors such as those published in ICRP report 74. This system consists of more than one neutron counter and does not directly make use of a moderator layer.
Two patents, one by Mikio (1991) and the other by Masahiro (2008), describe a thermal neutron detector embedded inside concentric hollow spherical shells. The energy response of both systems can be changed by filling or emptying the different hollow shells with moderating material. In the first of the two patents, the proposed moderating material is a liquid while in the second case it is powder. The shells are fixed, only their content is changed. The invention proposed herein does not call upon the filling and emptying of fixed shells and is more practical for use in an operational setting.
Bramblett et al. describe a neutron spectrometer commonly referred to as Bonner Sphere System. The full spectrum from 25 meV to 20 MeV is deduced from the count rates measured by a thermal neutron detector inside polyethylene spheres of radii from 3 inches to 15 inches. Typically 7 to 12 spheres are used. The user must insert the detector into each spheres in turn and takes as many measurements as there are spheres. From the energy spectrum, other quantities of interest, such as the dose equivalent, can be found using conversion factors such as those published in ICRP report 74. The Bonner Sphere System provides the most valuable information, for the following reasons: 1) it provides full energy spectra in the range of energies of 25 meV to 20 MeV, 2) it is a sensitive instrument that counts up to 1000 times faster than a proton recoil spectrometer. The disadvantages of the Bonner Sphere System are the following: 1) it is heavy: the full set of polyethylene spheres can weigh as much as 25 kg, 2) it is large: transport of the equipment may require 40 L of carrying capacity, 3) the data analysis is laborious: the conversion of the acquired data into neutron energy spectra requires the intervention of an expert user. The basic design of the Bonner Sphere system has not changed in over 45 years, as evidenced by a publication by Vega-Carrillo et al. However, recent work by Howell et al. has aimed to extend the sensitivity of the Bonner Sphere System to near 1 GeV by surrounding one of the moderating spheres with concentric shells of high atomic number material such as copper, tungsten and lead. A Bonner Sphere System with an extended energy range can be used in applications of neutron spectrometry in space, high altitude air travel and around particle accelerators.
Therefore, there is a need for a neutron spectrometer, wherein the spectra may be determined using a device that is less bulky, heavy and awkward than the “Bonner Spheres”-type spectrometer in the art, yet retaining the high sensitivity and wide energy response qualities which are so beneficial and attractive in this type of spectrometer.